Chemical Fate of Particulate Sulfur from Nighttime Oxidation of Thiophene

Sulfur-containing volatile organic compounds emitted during wildfire events, such as dimethyl sulfide, are known to form secondary aerosols containing inorganic sulfate (SO₄ ^2-) and surfactant-like organic compounds; however, little is known about the fate of sulfur in other emitted reduced organosulfur species. This study aimed to determine the sulfurous product distribution resulting from the nighttime oxidation of thiophene as a model system. Ion chromatography (IC) and aerosol mass spectrometry (a mini aerosol mass spectrometer, mAMS) were used to constrain the proportions of sulfurous compounds produced under wildfire-relevant conditions ([NO₂]/[O₃] = 0.1). With constraints from IC, results indicated that the sulfurous particle mass consisted of 30.3 ± 6.6% SO₄ ^2-, while mAMS fractionation attributed 24.5 ± 1.6% of total sulfate signal to SO₄ ^2-, 15.4 ± 1.9% to organosulfates, and 60.1 ± 0.9% to sulfonates. Empirical formulas of organosulfur products were identified as C1-C8 organosulfates and sulfonates using complementary mass spectrometry techniques. This study highlights the nighttime oxidation of thiophene and its derivatives as a source of SO₄ ^2- and particulate organosulfur compounds, which have important implications for the atmospheric sulfur budget and aerosol/droplet physical and chemical properties.